This section provides background information related to the present disclosure that is not necessarily prior art.
Shape memory alloy (SMA) actuators are used for a wide range of applications. One typically application for SMA actuators is to perform limited displacements which generate work. In such displacement applications, the SMA actuator is typically in the form of a wire that transforms linear motion into incremental relative motion. When applying a current to the cold (low temperature or martensitic state) shape memory alloy (SMA) wire the temperature rises until the transformation temperature is reached (high temperature or austenite state) and due to a crystalline restructuring of the material, a contraction occurs. With such a contraction, force or torque is thus generated. However, it is to be appreciated that after the contraction, the SMA wire does not reset itself and therefore a counterforce has to bring the SMA wire into its original position. As the SMA wire cools it returns to its low temperature or martensitic state. The temperature at which the shape memory alloy remembers its high temperature form, referred to as the phase transformation temperature, can be adjusted by applying stress and other methods. In nickel-titanium shape memory alloys, for example, it can be changed from above about 100° C. to below about −100° C.
SMA actuators can be slow to reset or experience unintended actuations when the ambient temperature approaches and exceed the phase transformation temperature. The stress applied to a shape memory alloy element may be increased to increase its phase transformation temperature. However, this increase of stress reduces the fatigue life of the shape memory alloy. Not only does continuously operating the shape memory alloy at a higher actuation temperature result in a much shorter life cycle, but a device that is designed form a higher ambient temperature environment has a much shorter life cycle even if it rarely experiences those higher ambient temperatures. An SMA actuator must be designed to balance competing interests of lower temperature actuation to allow a longer life cycle and actuator performance at elevated ambient temperatures.